1. Field of the Invention
The invention relates to a receiving method comprising a plurality of individual receiving antennas in which a phase modulation is superimposed on an antenna output signal, the antenna signals are summated and the summation signal amplitude demodulated. The invention also relates to a receiving antenna system for carrying out the method comprising a plurality of individual receiving antennas, a phase modulator, a summation circuit and an amplitude demodulator.
2. Description of the Related Technology
In mobile reception, for example reception of radio and/or television transmissions in motor vehicles, reception disturbances occur which considerably impair the reception. Such reception disturbances are due to the incidence of the radio or television waves on the antenna from more than one direction. This so called multipath reception occurs because the radio or television waves do not only reach the antenna directly from the transmitter but for example are reflected at buildings and also reach the receiving antennas along other paths. The reception paths for the plurality of signals received by the receiving antenna have different lengths so that in the radio and television signal, in particular with frequency-modulated carrier, interference disturbances occur, the resultant carrier thereby undergoing both an amplitude demodulation and a phase demodulation. The latter then given the annoying reception disturbances which considerably impair the reception and which due to the physical factors occur irrespective of the type of antennas employed, whether they are telescopic antennas, electronic short-rod antennas or electronic wind screen antennas.
In an article by R. Heidester & K. Vogt in NTZ 1958, No. 6, pages 315-319, a receiving antenna system is for example described which for reducing this interference occurring due to multipath reception comprises a plurality of individual receiving antennas for mobile reception. Associated with each individual antenna in this known arrangement is a receiver with which the amplitude of each individual signal of the respective individual antenna is continuously determined and monitored. The amplitudes determined are compared and the respective strongest signal of an individual antenna is used as reception signal. This type of diversity system, also referred to as parallel or receiver diversity system, is however very extensive and complicated in circuit technology because each antenna must be provided with a receiver. In addition, it is not certain that the individual antenna which furnishes the strongest antenna signal and is connected to the radio receiver in accordance with the aforementioned criterion necessarily furnishes the best signal, this being true in particular of frequency-modulated signals.
EP 0 201 977 A2, DE 3,334,735 A2 and the journal "Funkschau", 1986, pages 42-45, disclose for example a further receiving antenna system of the type mentioned at the beginning in which switching from one antenna to the other or from one linear combination of antenna voltages to other linear combinations is carried out when the reception quality drops below a predetermined threshold. This method, also known as scanning diversity or antenna selection diversity system, has however the substantial disadvantage that this switching operation is initiated only when interference has occurred. To achieve a transition between the antennas or linear combinations of antenna voltages which is satisfactory for the user and cannot be heard by him the switching over must take place extremely quickly and from the circuit technology point of view this is difficult, very complicated and then nevertheless only possible to a restricted extent. A further substantial disadvantage of this receiving system also resides in that an antenna furnishing a relatively poor reception signal which is however just beneath the switching threshold is kept in operation although other antennas furnish better reception signals with less interference. Furthermore, when interference occurs at the antenna which happens to be activated switching results to an arbitrarily selected following antenna or linear combination of antenna voltages which may also be disturbed or, as described above, lie just below the switching threshold. The reception properties of this diversity system are therefore not satisfactory.
DE 3,510,580 A1 discloses an antenna receiving method or system in which the phase of a reception signal is abruptly changed arbitrarily or statistically and the resulting amplitude change is measured. The setting of the optimum phase position in this case is by the "trial and error" principle in several steps. Thus, a phase change is initiated and it is determined whether this results in a better or worse total amplitude. The result is stored in each case in a processor. Thus, the "trial and error" method can at times also result in a deterioration of the reception conditions.
U.S. Pat. No. 4,079,318 discloses a known receiving method comprising a plurality of individual receiving antennas in which a phase modulation is superimposed on an antenna output signal by means of a phase modulator. In a summation circuit the phase-modulated output signal is added to the non-phase-modulated other antenna output signal. After frequency conversion and an intermediate frequency amplification in a conventional receiving circuit the summation signal is amplitude demodulated in a following amplitude detector and a synchronous detector. This gives a control signal with which a phase rotation element is continuously regulated so that the input signals of the summation circuit are brought to a uniform phase position.
this known method is intended for the transmission in the microwave range for reducing the socalled fading effects, the microwave transmission system receiving a single electromagnetic signal with two or more antennas. This known receiving method is thus intended for directional radio links and thus for stationary antennas. With this known control method the receiving lobes of directional radio antennas are caused to follow up to obtain an optimum reception with stationary receiving antennas. The follow-up takes place in minutes or hours, i.e. is relatively slow. There is no necessity for providing highspeed follow-up because the reception lobes, as stated, can change only slowly in said periods of time. An application of this known method to mobile radio, in particular the VHF radio reception in motor vehicles with a lowest transmitted frequency of 40 Hz, is thus not possible because in mobile reception follow-up times of less than 20 msec are necessary. The follow-up speed in mobile motor vehicle reception is thus several orders of magnitude faster. Moreover, the considerably longer follow-up times present in the known method can lead to noise in the audible range as will be explained below in detail regarding the prior art known from DE 3,510,580 A1.
A highspeed automatic control as is necessary for the reasons given in mobile radio diversity systems is not only unnecessary in stationary diversity systems but is impossible therein because of the interference which occurs. In the method known from U.S. Pat. No. 4,079,318 the one antenna output signal is phase-modulated with a low-frequency signal. The control must therefore necessarily take place slowly because with this low-frequency modulation the fluctuations of the interference quantities can be followed only with a relatively low speed. For this reason as well this known stationary diversity receiving method is not suitable for mobile radio diversity reception.
Furthermore, in the known method the frequency bandwidth for the baseband signal is also limited and the bottom frequency range cannot be utilized because of the low-frequency modulation signal. For this reason as well the known receiving method is not suitable for mobile radio diversity reception.
In the stationary diversity receiving systems in the microwave range the phase deviations due to the control operation then starting are relatively small and do not exceed 90.degree. phase deviation. As a result the control signals also only lie within a narrow signal or voltage range. The control signal always remains proportional to the phase deviation. The drop of the control signal occurring at very large phase deviations and the indifferent range occurring at 180.degree. phase deviation are not reached. In contrast, in mobile diversity systems large phase diviations and jumps extending over the entire phase range of 360.degree. and high change rates (Rayleigh fading) are to be expected. In this case a control method as described in EP-02 27 015 A2 for stationary diversity systems could not be used because of the slow response and also because the regulation would become even slower in the event of indifferences.
A further basic difference between diversity receiving methods for stationary directional radio transmissions and mobile reception in motor vehicles resides in particular also in that the individual antennas in directional radio systems in contrast to mobile radio diversity antennas for example in motor vehicles always receive a signal whereas in mobile radio due to the very different incidence angle and characteristics each antenna can receive the signal in any phase and amplitude position up to complete absence. Thus, whereas stationary antennas for optical alignment with the receiving lobe only need to follow up relatively slowly, in the case of mobile diversity systems it is necessary not only to detect and process phase changes but also amplitude changes in extremely wide change ranges. This is not possible with the receiving method known from EP 02 27 015 A2.